Biology subjects

Letcher, J. M.

Publications and source records attributed to Letcher, J. M..

4 recordsLinked to original sources

bioRxiv2021

Chloride-dependent mechanisms of multimodal sensory discrimination and neuropathic sensitization in Drosophila

Individual sensory neurons can be tuned to many stimuli, each driving unique, stimulus-relevant behaviors, and the ability of multimodal nociceptor neurons to discriminate between potentially harmful and innocuous stimuli is broadly important for organismal survival. Moreover, disruptions in the capacity to differentiate between noxious and innocuous stimuli can result in neuropathic pain. Drosophila larval Class III (CIII) neurons are peripheral noxious cold nociceptors and gentle touch mechanosensors; high levels of activation drive cold-evoked contraction (CT) behavior, while low levels of activation result in a suite of touch-associated behaviors. However, it is unknown what molecular factors underlie CIII multimodality. Here, we show that the TMEM16/anoctamins subdued and white walker (wwk; CG15270) are required for cold-evoked CT, but not for touch-associated behavior, indicating a conserved role for anoctamins in nociception. We also evidence that CIII neurons make use of atypical depolarizing chloride currents to encode cold, and that overexpression of ncc69--a fly homologue of NKCC1--results in phenotypes consistent with neuropathic sensitization, including behavioral hypersensitization and spontaneous nociceptor activity, making Drosophila CIII neurons a candidate system for future studies of the basic mechanisms underlying neuropathic pain.

neuroscience↗

bioRxiv2021

The evolution of cold nociception in drosophilid larvae and identification of a neural basis for cold acclimation

Cold temperatures can be fatal to insects, but many species have evolved the ability to cold acclimate, thereby increasing their cold tolerance. While there is a growing body of knowledge concerning the mechanisms underlying cold tolerance, relatively little is known concerning how insects sense noxious cold (cold nociception), or how cold nociception might function in cold tolerance. It has been previously shown that Drosophila melanogaster larvae perform highly stereotyped, cold-evoked behaviors under the control of noxious cold-sensing neurons (nociceptors) innervating the barrier epidermis. In the present study, we first sought to describe cold-nociceptive behavior among 11 drosophilid species with differing cold tolerances and from differing climates. Behavioral analyses revealed that the predominant cold-evoked response among drosophilid larvae is a head-to-tail contraction (CT) behavior, which is likely inherited from a common ancestor. However, despite lack of phylogenetic signal (suggesting trait lability), the CT behavior was transient and there was no clear evidence that cold sensitivity was related to thermal environment; collectively this suggests that the behavior might not be adaptive. We therefore sought to uncover an alternative way that cold nociception might be protective. Using a combination of cold-shock assays, optogenetics, electrophysiology, and methods to genetically disrupt neural transmission, we demonstrate that cold sensing neurons in Drosophila melanogaster (Class III nociceptors) are sensitized by and critical to cold acclimation. Moreover, we demonstrate that cold acclimation can be optogenetically-evoked, sans cold. Collectively, these findings reveal that cold nociception constitutes a peripheral neural basis for Drosophila larval cold acclimation. Significance StatementMany insects adapt to cold in response to developmental exposure to cool temperatures. While there is a growing body of knowledge concerning the mechanisms underlying cold tolerance, it is unknown how sensory neurons might contribute. Here, we show that noxious cold sensing (cold nociception) is widely present among drosophilid larvae, and that cold-sensing neurons (Class III cold nociceptors) are necessary and sufficient drivers of cold acclimation. This suggests that cold acclimation has, at least in part, a neural basis.

neuroscience↗

bioRxiv2019

Drosophila menthol sensitivity and the Precambrian origins of TRP-dependent chemosensation

Transient receptor potential (TRP) cation channels are highly conserved, polymodal sensors which respond to a wide variety of stimuli. Perhaps most notably, TRP channels serve critical functions in nociception and pain. A growing body of evidence suggests that TRPM (Melastatin) and TRPA (Ankyrin) thermal and electrophile sensitivities predate the protostome-deuterostome split (>550 million years ago). However, TRPM and TRPA channels are also thought to detect modified terpenes (e.g., menthol). Although terpenoids like menthol are thought to be aversive and/or harmful to insects, mechanistic sensitivity studies have been largely restricted to chordates. Furthermore, it is unknown if TRP-menthol sensing is as ancient as thermal and/or electrophile sensitivity. Combining genetic, optical, electrophysiological, behavioural, and phylogenetic approaches, we tested the hypothesis that insect TRP channels play a conserved role in menthol sensing. We found that topical application of menthol to Drosophila melanogaster larvae elicits a Trpm- and TrpA1-dependent nocifensive rolling behaviour, which requires activation of Class IV nociceptor neurons. Further, in characterizing the evolution of TRP channels, we put forth the hypotheses that 3 previously undescribed TRPM channel clades (basal, TRPM, and {beta}TRPM), as well as TRPs with residues critical for menthol sensing, were present in ancestral bilaterians.

neuroscience↗

bioRxiv2017

Formin3 regulates dendritic architecture via microtubule stabilization and is required for somatosensory nociceptive behavior

The acquisition, maintenance and modulation of dendritic architecture are critical to neuronal form, plasticity and function. Morphologically, dendritic shape impacts functional connectivity and is largely mediated by organization and dynamics of cytoskeletal fibers that provide the underlying scaffold and tracks for intracellular trafficking. Identifying molecular factors that regulate dendritic cytoskeletal architecture is therefore important in understanding mechanistic links between cytoskeletal organization and neuronal function. In a neurogenomic-driven genetic screen of cytoskeletal regulatory molecules, we identified Formin3 (Form3) as a critical regulator of cytoskeletal architecture in Drosophila nociceptive sensory neurons. Form3 is a member of the conserved Formin family of multi-functional cytoskeletal regulators and time course analyses reveal Form3 is cell-autonomously required for maintenance of complex dendritic arbors. Cytoskeletal imaging demonstrates form3 mutants exhibit a specific destabilization of the dendritic microtubule (MT) cytoskeleton, together with defective dendritic trafficking of mitochondria, satellite Golgi and the TRPA channel Painless. Biochemical studies reveal Form3 directly interacts with MTs via FH1-FH2 domains and promotes MT stabilization via acetylation. Neurologically, mutations in human Inverted Formin 2 (INF2; ortholog of form3) have been causally linked to Charcot-Marie-Tooth (CMT) disease. CMT sensory neuropathies lead to impaired peripheral sensitivity. Defects in form3 function in nociceptive neurons results in a severe impairment in noxious heat evoked behaviors. Expression of the INF2 FH1-FH2 domains rescues form3 defects in MT stabilization and nocifensive behavior revealing conserved functions in regulating the cytoskeleton and sensory behavior thereby providing novel mechanistic insights into potential etiologies of CMT sensory neuropathies.\n\nSignificance StatementMechanisms governing cytoskeletal architecture are critical in regulating neural function as aberrations are linked to a broad spectrum of neurological and neurocognitive disorders. Formins are important cytoskeletal regulators however their mechanistic roles in neuronal architecture are poorly understood. We demonstrate mutations in Drosophila formin3 lead to progressive destabilization of the dendritic microtubule cytoskeleton resulting in severely reduced arborization coupled to impaired organelle and ion channel trafficking, as well as nociceptive sensitivity. INF2 mutations are implicated in CMT sensory neuropathies, and INF2 expression can rescue microtubule and nociceptive behavioral defects in form3 mutants. While CMT sensory neuropathies have been linked to defects in axonal development and myelination, our studies connect dendritic cytoskeletal defects with peripheral insensitivity suggesting possible alternative etiological bases.

neuroscience↗